How to fix excessive/complex topology
Retopologizing is recommended often, but the truth is, retopologizing is an enormous amount of work, and many people lack the skills to do it properly. For this reason, there are often paid add-ons that help a lot, such as Retopo Flow. Assuming that isn't an option, there may be something you can do, but could be quite tricky, and there are no guarantees, since Blender has no dedicated and complete retopology solution right out of the box. But it does have the Remesh modifier. Admittedly, I haven't gotten that to work very well in my own work, largely due to me being baffled by all its cryptic settings. But I've had more luck with a newer option called Quad Flow Remesh. However, I only ever came across it in Blender 2.81, and even then, I had to use the search function (F3) to access it because it didn't yet have a location in the UI. I'm using 2.82a currently, and this feature doesn't seem to be accessible the same way. It might have been moved. I seem to recall finding a similar tool in the Active Tool properties panel while using sculpting brushes, but I can't be sure that was the same feature. If any one knows about this tool or its whereabouts, I'm all ears.